1. Field of the Invention
The present invention relates to an internal combustion engine in which a high RON fuel and a low RON fuel are mixed and fed to a combustion chamber, wherein high RON fuel means high octane number fuel, and low RON fuel means low octane number fuel.
2. Description of the Related Art
The low RON fuel has a good ignitability and a poor antiknock property, and the high RON fuel has a poor ignitability and a good antiknock property. Accordingly, an internal combustion engine in which the low RON fuel is stored in a low RON fuel tank and the high RON fuel is stored in a high RON fuel tank, and the low RON fuel and the high RON fuel are fed to a combustion chamber at a mixing ratio appropriate to a driving condition is well known and is disclosed in, for example, Japanese Unexamined Patent Publication (Kokai) No. 2001-50070.
In the internal combustion engine described in Japanese Unexamined Patent Publication (Kokai) No. 2001-50070, a target fuel mixing ratio is determined based on a running condition and fuel volumes in each tank. Multiple kinds of fuels are injected from a fuel injector so that the determined target fuel mixing ratio is achieved. However, the fuel infected from the fuel injector can stick to an intake port and, accordingly, a divergence, between the mixing ratio of the fuel actually fed to a combustion chamber and the target fuel mixing ratio, occurs. On the other hand, an ignition timing is set on the precondition that a plurality of fuel components are fed at the target fuel mixing ratio. Therefore, if a divergence, between the mixing ratio of the fuel actually fed to a combustion chamber and the target fuel mixing ratio, occurs a predetermined performance cannot be achieved.